Red tides are not unusual in the waters of the Gulf of Mexico off the western coast of Florida. The strong smell; eye, nose, and throat irritation; and large fish kills related to the event have been documented as far back as the 1840s. Red tides are caused by tiny algae that grow on the surface of the ocean, occasionally giving it a reddish-brown tint. Thus, scientists can use satellite imagery to map the extent of red tides and monitor how they spread over time. Satellites detect changes in the way the sea surface reflects light. These changes can be linked to concentrations of chlorophyll, showing where algae and other ocean plants are concentrated in the ocean.

In mid-November 2004, scientists began to notice an algae bloom developing in the Gulf of Mexico, and ground tests confirmed the presence of red tide. By December 8, the bloom had spread to cover 400 square miles. The images above show chlorophyll concentrations in the Gulf of Mexico off southwestern Florida on October 30 (bottom right corner) and November 21, 2004 (left), as well as chlorophyll fluorescence (upper right) on November 21. Highest concentrations of chlorophyll and highest levels of fluorescence are red; lower values are green and blue.

The red tide is clearly visible as the oval-shaped red area to the west of the shore in the November 21 image from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) flying on the OrbView-2 satellite. The high chlorophyll concentrations occur between Charlotte Harbor and the Florida Keys, which matches the location of the bloom.

A challenge scientists face when interpreting satellite images of red tides is that what may appear to be high levels of chlorophyll could in fact be chlorophyll and something else. Shallow coastal areas are rich in sediment and organic matter deposited by rivers and stirred up by tides. So chlorophyll may be present, but it is mixed in with these other substances that influence the color and intensity of the light reflected by the ocean. This effect is visible in the image from October 30. Though the obvious sign of the red tide had not yet developed — note the red bulge detected offshore on November 21 is missing — coastal waters were still reflective enough to suggest high chlorophyll concentrations along the coast.

One way to determine whether a satellite has detected sediment and organic matter or chlorophyll is to look at fluorescence signals. When algae absorb light, not all of it is converted to energy; some is converted to heat, and some is released as light. The re-emitted light, called fluorescence, is not the same wavelength as sunlight that is simply reflected by the surface. The upper-right image shows fluorescence measured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite on November 21. Areas with high fluorescence, measured in power (Watts) per area (square meters) per wavelength (micrometer) per solid angle (steradian), are red and orange, while lower fluorescence values are blue. The region of highest fluorescence corresponds well with the bulge of high chlorophyll concentrations recorded by SeaWiFS on the same day, providing further evidence that the feature is in fact the ongoing red tide.

The event is being closely monitored because of the deadly effect the plant has on marine life. Though shellfish are unaffected, the toxin produced by the algae can collect in their flesh, making them harmful to anyone who eats them. When the plants drift close to shore, the toxins can irritate the respiratory system and cause eye, nose, and throat irritation. So far, this red tide has killed a number of fish, crabs, and a few dolphins near the Florida Keys. As of December 9, it had not reached the Florida coast, and current and wind patterns were expected to push the bloom southward, away from the shore.